Field of the Invention
The present invention relates a liquid crystal display (LCD) device, and more particularly, to an apparatus and method for driving an LCD device to provide improved picture quality.
Discussion of the Related Art
Generally, LCD devices adjust light transmittance of liquid crystal cells according to a video signal to display an image. An active matrix type LCD device, which has a switching element for every liquid crystal cell, is suitable for the display of a moving image. A thin film transistor (hereinafter, referred to as a TFT) is mainly used as the switching element in the active matrix type LCD device.
However, the LCD device has a relatively low response speed due to characteristics such as the inherent viscosity and elasticity of liquid crystal, as can be seen from the following equations 1 and 2:
                              τ          r                ∝                              γ            ⁢                                                  ⁢                          d              2                                            Δɛ            ⁢                                                                          V                  a                  2                                -                                  V                  F                  2                                                                                                      [                  Equation          ⁢                                          ⁢          1                ]                            where τr is a rising time when a voltage is applied to the liquid crystal, Va is the applied voltage, VF is a Freederick transition voltage at which liquid crystal molecules start to be inclined, d is a liquid crystal cell gap, and y is the rotational viscosity of the liquid crystal molecules.        
                              τ          F                ∝                              γ            ⁢                                                  ⁢                          d              2                                K                                    [                  Equation          ⁢                                          ⁢          2                ]                            where τF is a falling time when the liquid crystal is returned to its original position because of an elastic restoration force after the voltage applied to the liquid crystal is turned off, and K is the inherent elastic modulus of the liquid crystal.        
In a twisted nematic (TN) mode, although the response speed of the liquid crystal may be different according to the physical properties and cell gap of the liquid crystal, it is common that the rising time is 20 to 80 ms and the falling time is 20 to 30 ms. Because this liquid crystal response speed is longer than one frame period (16.67 ms in National Television Standards Committee (NTSC)) of a moving image, the response of the liquid crystal proceeds to the next frame before a voltage being charged on the liquid crystal reaches a desired level, as shown in FIG. 1, resulting in motion blurring in which an afterimage is left in the eyeplane.
With reference to FIG. 1, a related art LCD device cannot express a desired color and brightness for display of a moving image in that, when data VD is changed from one level to another level, the corresponding display brightness level BL is unable to reach a desired value due to a slow response of the liquid crystal display device. As a result, the motion blurring occurs in the moving image, causing degradation in contrast ratio and, in turn, degradation in display quality.
In order to solve the low response speed of the liquid crystal display device, U.S. Pat. No. 5,495,265 and PCT International Publication No. WO 99/09967 has proposed a method for modulating data according to a variation therein using a look-up table (referred to hereinafter as an ‘over-driving method’). This over-driving method is adapted to modulate data on the basis of a principle as illustrated in FIG. 2.
With reference to FIG. 2, the related art over-driving method includes modulating input data VD and applying the modulated data MVD to a liquid crystal cell to obtain a desired brightness level MBL. In this over-driving method, in order to obtain the desired brightness level corresponding to the luminance of the input data in one frame period, the response of a liquid crystal is rapidly accelerated by increasing |Va2−VF2| Equation 1 on the basis of a variation in the input data.
Accordingly, a related art liquid crystal display device using the over-driving method is able to compensate for a slow response of a liquid crystal by modulation of a data value to relax motion blurring in a moving image to display a picture with a desired color and brightness.
For this, a related art over-driving circuit includes a frame memory 302 connected to a bus line 301, and a look-up table 303 connected in common to output terminals of the bus line 301 and the frame memory 302, as illustrated in FIG. 3.
The frame memory 302 stores data (RGB) from the bus line 301 for one frame period and supplies the stored data (RGB) to the look-up table 303. The look-up table 303 compares data (RGB) of a current frame (Fn) from the bus line 301 with data (RGB) of a previous frame (Fn−1) from the frame memory 302, and selects modulated data (MRGB) corresponding to the comparison result.
However, the related art over-driving method has the following disadvantages.
In the related art over-driving method, in case of 8-bit data, it cannot be driven by a voltage which is higher than a value corresponding to gray scale 255 of the uppermost gray scale. Accordingly, if the gray scale is changed from gray 0 of the lowermost gray scale to gray 255, the LCD device is driven by the voltage corresponding to the gray scale 255 without modulating a data voltage to a higher voltage than the value corresponding to the gray scale 255.
In the related art over-driving method, it is difficult to obtain the rapid response of liquid crystal for the lowermost or uppermost gray scale. Thus, it is difficult to improve the picture quality.